This invention relates generally to electronic test equipment, and more particularly to a self-contained hand-held electronic test probe for trouble shooting DC electrical systems having maximum test voltages in the range of 6-24 volts.
In trouble shooting DC electrical systems, in particular automotive type systems, it is desirable to have an easy to use portable hand-held probe which includes a probe member for making electrical contact at selected points throughout the electrical system under inspection and which also has a visual indicator which provides an immediate indication of the signal received by the probe member. With the visual indication capability as a part of the self-contained hand-held probe, an operator can rapidly test the electrical system without having his attention diverted from the system by the necessity of referring to a remotely located indicator or panel. It is also desirable to have a self-contained probe which has its own source of power such that external power supplies are unnecessary. Additionally, it is desirable to have a probe that will provide an indication of ground potential only for low impedance ground (i.e., typically less than 100 ohms), since high impedance grounds are particularly detrimental in automotive electrical systems.
Test probes having one or more of these features are known in the prior art. However, most prior art test probes include a plurality of indicator lamps in a hand-held probe unit for indicating positive or negative voltage and which operate such that both indicator lamps are normally illuminated, and wherein one of the lamps is extinguished when a particular voltage signal is received by the probe. In the operation of such devices, both of the visual indicators must be illuminated throughout substantially the entire test procedure, thus reducing the operative life of the probe and making such circuit construction generally impractical for use in self-contained battery powered probes. Additionally, in some such devices, substantially all of the measured voltage is applied across the appropriate visual indicator subjecting the indicator to severe transient conditions, thus further reducing their operative life. Several of the prior art test probes incorporate transistor components having input impedances on the order of 5000 ohms. Thus, grounds having impedances as high as a few hundred ohms will provide an indication of a "good" ground when, in fact, the ground would be unacceptable in an automotive electrical system.
In the prior art test probes which included self-contained batteries for energizing the test circuits, the devices are generally limited in their application and have significant non-operative current drains requiring frequent maintenance and replacement of the batteries.
The inventor of the present invention in a previous application, Ser. No. 642,077, filed Dec. 18, 1975, now U.S. Pat. No. 4,028,621 disclosed an inexpensive and easy to use test probe that incorporated a unique circuit design for maximizing reliability and life of the individual probe components. The applicant's previous disclosure, however, is designed solely for electrical systems having a single predetermined DC voltage level. Additionally, the inventor's prior test probe required the use of an external source of power, typically the battery of the automobile. Due to the circuit construction of the inventor's prior probe structure, that test probe could not be used for testing DC electrical systems having widely varying test voltage ranges, such as tractors or airplanes having dual voltage (i.e., 12/24 volts) systems as well as the 6 volt systems of older models of farm tractors and motorcycles. Additionally, the inventor's previous test probe cannot be used to check the electrical systems of vehicles which do not have an operating battery since an external source of power is required.
The present invention eliminates these disadvantages of the prior art in that it includes a self-contained hand-held pocket probe capable of testing DC electrical systems over a wide range of maximum voltages (i.e., 6 volts to 24 volts) and is capable of measuring relatively low-impedance ground connections. The test probe of the present invention incorporates electrical circuits which limit the maximum current through the probe illumination devices, thereby extending the reliability and operating life of the probe. Additionally, the quiescent current drain on the self-contained battery of the probe is minimal such that the life of the batteries utilized is maximized, requiring low maintenance and less frequent replacement. The probe is sized to be easily carried within the shirt pocket of the user and incorporates a single elongated flexible conductor extending therefrom for electrical connection to a known ground potential of the circuit being tested.
While particular probe housing and ground connector configurations will be disclosed, it is understood that other configurations could equally well be employed within the spirit and scope of the present invention. Further, while specific values of the circuit elements are disclosed with respect to the preferred embodiments of the electrical circuitry of the present invention, it is apparent that an operatively equivalent test probe circuit could be designed using somewhat different component values for the inventive circuit configurations disclosed herein.